1. Field of the Invention
The present invention relates to circuits for driving gas discharge lighting circuits. More particularly, the present invention relates to a protection circuit for an electronic ballast or other power supply circuit that detects an arc somewhere in the system and shuts down the ballast.
2. Problems in the Art
A potential fire hazard exists when arcing occurs in an electronic circuit. This hazard is much greater when high frequency electronic ballasts are used. Arcing is usually caused by poor connections in the wiring of fluorescent lamps or other discharge lamps that have high open circuit voltages. Arcing can occur at any poor connection where an effective air gap is created such as at the lamp socket or at an in-line connector. If there is an air gap, arcing will occur if the lamp voltage and current are high enough. Typically, in a line frequency fluorescent lamp circuit, an arc will extinguish itself as the alternating current passes through the zero axis. However, in a high frequency electronic ballast lamp circuit, the arc can be maintained through the current zero crossing if there is not sufficient de-ionization time for the arc to extinguish during the current zero axis crossing. The energy in an arc causes a rise in temperature and a potential out-gassing and combustion of adjacent flammable materials. This may result in burned lamp sockets or connectors.
The heat energy resulting from an arc of a typical constant current high frequency electronic ballast will equal the lamp current times the voltage drop across the arc. As a result, a large number of watts can be generated in a confined arc within a plastic connector or lamp socket. This large number of watts results in very high temperatures within the plastic connector or lamp socket. Unless the arc extinguishes itself, the high temperature created at the electrodes of the arc can cause the combustion of flammable adjacent material such as the plastic housing or electrical connector.
There are several prior art systems used to detect abnormal conditions in a circuit. For example, one prior system uses a current sensor along with a square wave generator and a counter to detect an arc across a break in a line. Other systems use various means to detect problems in a circuit such as a load failure condition, or a ground fault condition. These systems typically are complex and have a limited usefulness.
Therefore, a need can be seen for a system to eliminate the creation of high temperatures by an arc. Such a system would detect an arc in the circuit, latch the power supply off before high temperatures can develop across the arc, and force the circuit to remain off as long as desired. It is also desirable to provide a protection circuit that is simple, efficient, and works with most power supply circuits.